1. Field of the Invention
The present invention relates to a cylindrical magnetic resonance antenna which is embodied as a local antenna and is used for the reception of signals and/or for field generation. The invention also relates to a magnetic resonance device in which an antenna of this type is integrated.
2. Description of the Prior Art
A magnetic resonance antenna is known that has a number of longitudinal conductors extending in a longitudinal direction of the magnetic resonance antenna and are arranged such that they are separated from each other and aligned in parallel with each other in the circumferential direction. Two end rings which connect the ends of the longitudinal conductors and proceed along the circumferential direction.
The longitudinal conductors are one or more longitudinal conductor sections that are arranged one behind the other in a longitudinal direction, with at least one longitudinal capacitance, this being essentially identical for each longitudinal conductor, being connected between two immediately consecutive longitudinal conductor sections and/or between at least one longitudinal conductor section adjacent to the end ring and the end ring.
Two adjacent longitudinal conductors and the end ring segments that connect them form a network in each case, and the end ring segments connecting two adjacent longitudinal conductors in each have at least two end ring sections, between which is connected at least one end ring capacitance, this being essentially identical for all end ring segments.
A magnetic resonance antenna of this type is known as a “birdcage antenna”. In this context the two circular end rings are of coaxial design and are connected to each other by means of the longitudinal conductors, thereby suggesting the structure of a birdcage.
Such a magnetic resonance antenna has half as many different resonance frequencies as it has longitudinal conductors. A different resonance mode is assigned to each resonance frequency, the magnetic resonance antenna always being operated or controlled in that resonance mode in which a maximally homogeneous field distribution is produced within the antenna if the antenna is operated as a transmit antenna, or a location-independent constant sensitivity is present in the case of operation as a receive antenna.
In the case of such a magnetic resonance antenna, the homogeneity in its interior is usually calculated without further interfering elements. The homogeneity is disrupted, for example, as soon as a patient or part of a patient is situated within the magnetic resonance antenna. A possibility for correcting the homogeneity is therefore desirable. Furthermore, a number of location-dependent operating methods are known for magnetic resonance antennas. RX-Sense and TX-Sense are examples. In the case of RX-Sense, measurements are locally dispersed, with the location information of the magnetic resonance antenna being taken into consideration as well as the location information which is obtained from the gradient fields. In the case of TX-Sense, only a specific target region such as e.g. the heart must be stimulated. However, such methods require the provision of partial antennas of a magnetic resonance antenna which are able to be activated or read out independently in order to obtain the location information or transmit the location coding.
DE 197 02 256 A1 discloses a birdcage magnetic resonance antenna by means of which it is possible to improve the signal-to-noise ratio, i.e. a magnetic resonance antenna for receiving signals. To this end, it provides for the longitudinal capacitances and the end ring capacitances to be dimensioned such that the magnetic resonance antenna has only one single resonance frequency for all resonance modes. Consequently, the individual networks which form the magnetic resonance antenna are decoupled from each other such that the signals received from the individual networks can be processed independently to form magnetic resonance images which can be reassembled subsequently. However, this magnetic resonance antenna has the serious disadvantage that influences on the environment, e.g. adjacent gradient coils and their screening, can influence the arrangement such that no further matching resonance frequencies are present or the reception quality suffers considerably.
An arrangement for generating high-frequency fields in the examination volume of a magnetic resonance device is disclosed in DE 101 24 465 A1. This document also discloses a whole-body coil in the form of a birdcage coil. It is also proposed that the conductor elements of the whole-body coil be arranged at a small distance from the high-frequency screen of the magnetic resonance device, since this improves the decoupling characteristics of the whole-body coil.